1. Field of Invention
The present invention relates to a process for preparing ammonia and a substantially ammonia-free solution of sodium orthophosphate by reacting aqueous monoammonium phosphate with sodium carbonate in a vertical column having vapor-liquid contact means to form a solution of sodium ammonium phosphate having reduced carbon dioxide levels and thereafter subjecting the sodium ammonium phosphate solution to treatment in a high pressure/temperature column having vapor-liquid contact means to form ammonia and a substantially ammonia-free solution of sodium orthophosphate.
2. Description of the Prior Art
In the prior art, alkali metal phosphates such as monosodium phosphate, disodium phosphate, and trisodium phosphate, solutions thereof and compounds derived therefrom have been commercially prepared by neutralizing furnace grade or wet process phosphoric acid at atmospheric pressure with an alkali metal hydroxide or carbonate such as sodium or potassium hydroxide or carbonate and then further treating the product to form a desired phosphate, polyphosphate, or pyrophosphate.
In such processes an ammonium phosphate is not formed as an intermediate for at least two reasons. Ammonia required for the preparation of an ammonium phosphate intermediate is costly and generally not readily available at the phosphate production site. Secondly, an economical process for utilizing a route involving an ammonium phosphate intermediate would require an efficient, inexpensive method for recovering ammonia for sale or recycle to the process and a suitable ammonia recovery process was not heretofore available.
It is generally known that the degree to which ammonia can be recovered from an aqueous system is a function of the alkalinity of the system. Thus, in the Kjeldahl method for analyzing for ammonia, highly alkaline solutions are employed in order to facilitate the release of ammonia. Likewise, in urea technology use of highly alkaline solutions facilitates substantially complete recovery of ammonia but generally only in combination with carbon dioxide.
In aqueous phosphatic systems the ability to remove ammonia is likewise dependent on the alkalinity of the solution. Thus, at atmospheric pressure complete recovery of ammonia from an ammonium phosphate solution is only feasible in the presence of a high concentration of a base such as sodium hydroxide, for example, in amounts sufficient to provide an Na/P molar ratio of 3 or more. As one decreases the concentration of base, recovery of ammonia becomes less complete and ammonia recovery times increase dramatically. As the Na/P molar ratio is decreased from 3 or more to about 2 or less, ammonia recovery becomes uneconomical due to low yields and extended recovery time.
It is self-defeating to utilize a high Na/P molar ratio to facilitate ammonia recovery during alkali metal phosphate formation due to the fact that the high Na/P molar ratio merely limits the variety of phosphates which can be ultimately formed. For example, if an Na/P molar ratio of 2 or more is utilized to form an orthophosphate solution from an ammonium phosphate, the orthophosphate cannot advantageously be used to form such relatively acidic products as NaH.sub.2 PO.sub.4, Na.sub.2 H.sub.2 P.sub.2 O.sub.7 and (NaPO.sub.3).sub.x since each require an Na/P molar ratio of 1. Likewise, an orthophosphate solution having an Na/P molar ratio of 2 or more cannot advantageously be utilized as a source material for sodium tripolyphosphate which requires an Na/P molar ratio of about 1.667.
It is thus apparent that if substantially all ammonia could be recovered from relatively acidic ammonium phosphate solutions, for example, those having an alkali metal/P molar ratio below about 2 and preferably as low as about 1, the resulting orthophosphate could advantageously be employed to prepare all phosphates, polyphosphates, and pyrophosphates in which the ratio of alkali metal to P is 1 or more.
It is known, of course, to prepare either monoammonium phosphate or sodium ammonium phosphate from phosphoric acid and there are several patents dealing with processes for accomplishing this and for producing ammonium phosphates of such purity that they can be utilized to prepare high quality alkali metal phosphates.
It is also known to react monoammonium phosphate with sodium hydroxide to produce sodium ammonium phosphate in accordance with the equation: EQU I NH.sub.4 H.sub.2 PO.sub.4 + NaOH.fwdarw.NaNH.sub.4 H.sub.2 PO.sub.4 + H.sub.2 O (I)
additionally, it is known to prepare sodium ammonium phosphate according to the following equation: EQU II 2NH.sub.4 H.sub.2 PO.sub.4 + Na.sub.2 CO.sub.3 .fwdarw.2NaNH.sub.4 HPO.sub.4 + H.sub.2 O + CO.sub.2 (II)
where it is desired to employ the solution formed according to equation I or II as an intermediate in the preparation of sodium phosphates, equation I would appear to be the route of choice in that it circumvents problems which may arise during a subsequent ammonia recovery step due to the presence of carbon dioxide, carbonates, or bicarbonates. While this would appear to dictate in favor a route utilizing only caustic soda the differential between the price of caustic soda and the price of soda ash or trona as a source of sodium carbonate would dictate in favor of the route of equation II if carbon dioxide could be economically and effectively removed prior to ammonia recovery.
The separation of carbon dioxide from ammonia, however, is notoriously difficult and at best would require one or more additional processing steps. No means has previously been found for thermally separating carbon dioxide from an ammonium phosphate containing reaction mixture without simultaneously separating, and thus losing, a substantial amount of ammonia. If, for example, one attempts to heat the mixture or conduct the reaction at elevated temperature up to about 40% of the ammonia will be expelled with the carbon dioxide. This in and of itself represents a substantial loss of ammonia and to avoid this loss, an additional separating step would be required. Furthermore, as the carbon dioxide/ammonia mixture cools, carbamates may form, precipitate, and plug process lines. The formation of carbamates represents an unwarranted loss of ammonia and also increases corrosion in process lines and processing equipment and thus substantially decreases its useful life.
In addition, the presence of excessive amounts of carbon dioxide during a high temperature ammonia recovery step will complicate ammonia recovery and/or contaminate either the ammonia or the solution of sodium ammonium phosphate or both.
In order to avoid these losses and complications and to take advantage of the price differential between soda ash and caustic soda it is the principal object of the present invention to provide a process for conversion of an aqueous monoammonium phosphate to a substantially ammonia-free solution of sodium orthophosphate wherein carbon dioxide and ammonia are each separately and economically recovered.